The present invention relates to the field of mass storage devices. More particularly, this invention relates to a system and method for measuring drive-level bearing friction.
One key component of any computer system is a device to store data. Computer systems have many different places where data can be stored. One common place for storing massive amounts of data in a computer system is on a disc drive. The most basic parts of a disc drive are an information storage disc that is rotated, an actuator arm assembly including the transducer moves the transducer to various locations over the disc, and electrical circuitry that is used to write and read data to and from the disc. The disc drive also includes circuitry for encoding data so that it can be successfully retrieved and written to the disc surface. A microprocessor controls most of the operations of the disc drive as well as passing the data back to the requesting computer and taking data from a requesting computer for storing to the disc.
The transducer is typically placed on a small ceramic block, also referred to as a slider that is aerodynamically designed so that it flies over the disc. The slider is passed over the disc in a transducing relationship with the disc. Most sliders have an air-bearing surface (ABS) which includes rails and a cavity between the rails. When the disc rotates (typically at rotational speeds of 10,000 RPM or higher), air is dragged between the rails and the disc surface causing pressure, which forces the head away from the disc. At the same time, the air rushing past the cavity or depression in the air bearing surface produces a negative pressure area. The negative pressure or suction counteracts the pressure produced at the rails. The slider is also attached to a load spring, which produces a force on the slider directed toward the disc surface. The various forces on the slider equilibrate, so that the slider flies over the surface of the disc at a particular desired fly height. The fly height is the distance between the disc surface and the transducing head, which is typically the thickness of the air lubrication film. This film eliminates the friction and resulting wear that would occur if the transducing head and disc were in mechanical contact during disc rotation. In some disc drives, the slider passes through a layer of lubricant rather than flying over the surface of the disc.
Information representative of data is stored on the surface of the storage disc. Disc drive systems read and write information stored on tracks on storage discs. Transducers, in the form of read/write heads attached to the sliders, located on both sides of the storage disc, read and write information on the storage discs when the transducers are accurately positioned over one of the designated tracks on the surface of the storage disc. The transducer is also said to be moved to a target track. As the storage disc spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto a track by writing information representative of data onto the storage disc. Similarly, reading data on a storage disc is accomplished by positioning the read/write head above a target track and reading the stored material on the storage disc. To write on or read from different tracks, the read/write head is moved radially across the tracks to a selected target track. The data is divided or grouped together on the tracks. In some disc drives, the tracks are a multiplicity of concentric circular tracks. In other disc drives, a continuous spiral is one track on one side of the disc drive. Each track on a disc surface in a disc drive is further divided into a number of short arcs called sectors. Servo feedback information is used to accurately locate the transducer head onto the tracks/sectors. The actuator arm assembly is moved to the required position and held very accurately during a read or write operation using the servo information.
The actuator arm assembly is rotatably attached to a shaft via a bearing cartridge, which typically includes one or more sets of ball bearings. The shaft/post is attached to the base and may be attached to the top cover of the disc drive. A yoke is attached to the actuator arm assembly. The voice coil is attached to the yoke at one end of the rotary actuator arm assembly. The voice coil is part of a voice coil motor, which is used to rotate the actuator arm assembly including the attached transducer or transducers. A permanent magnet is attached to the base and cover of the disc drive. The voice coil motor, which drives the rotary actuator arm assembly, comprises the voice coil and the permanent magnet. The voice coil is attached to the rotary actuator arm assembly and the permanent magnet is fixed on the base. A yoke is typically used to attach the permanent magnet to the base and to direct the flux of the permanent magnet. Since the voice coil sandwiched between the magnet and yoke assembly is subjected to magnetic fields, electricity can be applied to the voice coil to drive it so as to position the transducers at a target track.
Tribological qualifications of the head-disc interface (head-actuator arm-shaft-bearing cartridge-disc interface) is generally a critical path to final qualification of the both the head and the disc during the design and development stages of new disc drives. This is because, the bearing cartridge is the only dynamic structure holding between the actuator arm assembly and the base in a disc drive. Generally, too much bearing friction in the bearing cartridge can cause seeking and settling problems in the actuator arm assembly, and too little bearing friction in the bearing cartridge can cause oscillations in the actuator arm assembly during track following. Therefore, it is critical to be able to characterize the drive-level bearing friction during disc drive qualifications. Current methods to measure bearing friction are performed at a component level using a dedicated bearing cartridge torque tester. Measuring bearing friction at the component level is a static bearing friction measurement. These static measurements generally do not reveal true drive-level bearing friction (dynamic bearing friction developed during operation of the disc drive). This is because relative position and loading conditions of the ball bearing raceways will be different in the disc drive during operation of the disc drive. Therefore, current methods using component level bearing friction measurements do not measure true dynamic bearing friction (drive-level bearing friction). Also, knowing drive-level bearing friction helps in better characterizing the bearing and further aids in analyzing problem disc drives.
What is needed is a system and method to measure bearing friction of a disc drive during operation of the disc drive that aids in effectively evaluating the disc drives.
In a first preferred embodiment, a disc drive includes a base and a disc rotatably attached to the base. The disc drive also includes a shaft attached to the base. A bearing cartridge is disposed in the base. An actuator arm assembly is attached to the shaft via the bearing cartridge such that the actuator arm assembly is in an actuating relationship with respect to the disc. The disc drive further includes a voice coil motor. The voice coil motor is coupled to the actuator arm assembly to position the actuator arm assembly including a transducer head over the rotating disc during track follow-and-seek operations of the disc drive. The voice coil motor is further coupled to a disc drive controller to control the movement of the actuator arm assembly. The disc drive controller includes a servo controller, an analyzer, a comparator, and a memory.
In a second preferred embodiment, the servo controller through the voice coil motor moves the actuator arm assembly to predetermined cylinder position intervals from an outermost cylinder position to an innermost cylinder position on the rotating disc. Then the servo controller measures the voice coil motor current required to move the actuator arm assembly to each of the predetermined cylinder position intervals. Then the servo controller further moves the actuator arm assembly to the predetermined cylinder position intervals from the innermost cylinder position to the outermost cylinder position on the rotating disc with an increment cylinder set by the user. Then the servo controller again measures the voice coil motor current required to move the actuator arm assembly to each of the predetermined cylinder position intervals.
In a third preferred embodiment, the analyzer then computes torque values required to move the actuator arm assembly to each of the predetermined cylinder position intervals using the measured voice coil motor currents. Then the analyzer computes drive-level bearing friction values to move the actuator arm assembly to each of the predetermined cylinder position intervals using the computed torque values. The analyzer further computes an average drive-level bearing friction value from the computed drive-level bearing friction values to determine the drive-level bearing friction of the disc drive. The comparator coupled to the analyzer compares the determined drive-level bearing friction with a predetermined drive-level bearing friction value to characterize the disc drive.
A fourth preferred embodiment is a method of measuring drive-level bearing friction and characterizing the disc drive using the measured drive-level bearing friction. The method begins with the step of measuring voice coil motor current values for moving an actuator arm assembly from an outermost cylinder position to each predetermined cylinder position interval until the actuator arm assembly reaches the innermost cylinder position. The method further requires measuring voice coil motor current values for moving the actuator arm assembly from the innermost cylinder position to each of the predetermined cylinder intervals until the actuator arm assembly reaches the outermost cylinder position.
A fifth embodiment comprises computing torque values required to move the actuator arm assembly to each of the predetermined cylinder position intervals from the measured voice coil motor currents. Then the method requires computing drive-level bearing friction values using the computed torque values. Then the method further requires computing an average drive-level bearing friction from the computed drive-level bearing friction values to determine the drive-level bearing friction of the disc drive. Next, the method comprises comparing the determined average drive-level bearing friction to a predetermined drive-level bearing friction to characterize the disc drive.
Further features and advantages of the present invention will become apparent upon a review of the following figures and their accompanying detailed description.